Deposition process for coating or filling re-entry shaped contact holes

ABSTRACT

A method and apparatus for depositing material to conformally cover or fill holes within the surface of a semiconductor substrate. The preferred method includes the steps of coherently depositing a first thickness of the material onto the surface of the substrate; reverse sputtering the deposited material so as to coat the sidewalls of the contact holes with the deposited material; after the first thickness of the material is deposited onto the surface of the substrate, depositing a second thickness of the material onto the surface of the substrate; and while depositing the second thickness of the material onto the surface of the substrate, heating the substrate to enhance reflow of the material being deposited.

This is a divisional of U.S. application Ser. No. 08/867,276, filed Jun.2, 1997, now U.S. Pat. No. 5,780,357, which is a continuation of Ser.No. 08/356,928, filed Dec. 14, 1994, now abandoned.

BACKGROUND OF THE INVENTION

The invention relates to methods and apparatus for sputter deposition ofmaterial into high aspect ratio, reentry shaped contact holes insemiconductor substrates. More specifically, the invention relates toconformally covering the walls of such holes with the depositedmaterial, or filling the holes with the material.

Semiconductor devices are typically multilayered structures fabricatedon semiconductor wafers one layer at a time. At least one of the layersof the multilayered structure is a metalization layer which is patternedto produce conductive pathways or wires that electrically interconnectthe devices that have been formed on the wafer. The metalization isusually deposited onto a passivation layer (e.g., SiO₂) that previouslywas deposited over the surface of the wafer to protect and isolate theunderlying devices. To enable the metalization layer to make electricalcontact to the semiconductor devices under the passivation layer,contact holes or vias are etched through the passivation layer prior todepositing the metal later. When the metal (e.g., Al) is deposited ontothe wafer, it fills the contact holes and makes contact with theunderlying devices.

It is generally desirable for contact holes to have vertical sidewalls.This is particularly true for the high aspect ratio contact holes (i.e.,high length-to-width ratio), which now are commonly used to fabricatesemiconductor devices with dimensions in the submicron range. However,vertical sidewalls are not always possible because the plasma etchprocesses that commonly are used to form the contact holes are difficultto control. If the plasma etch process is not stable, as often can bethe case, the layer being etched can be "undercut" so as to produce"re-entry shaped" holes (see FIG. 1). In a "reentry shaped" hole 2, thewalls 4 are not vertical as would be ideal; rather, the holeprogressively increases in width from the top (i.e., the opening) to thebottom of the hole.

A reentry shaped hole is considerably more difficult to coat or fillwith metal 6 than is a contact hole with ideally vertical sidewalls. Ifa conventional Al sputter deposition is used to coat or fill the hole,the deposited material will tend to build up near the upper edge of thecontact holes to form an overhang 8. The overhang will prevent sputteredmaterial from going into and reaching the bottom of the contact hole.For very small, high aspect ratio holes this problem is particularlysevere.

One can use a coherent sputter deposition to prevent the formation ofthe overhang. By coherent deposition, we mean that the sputteredmaterial that reaches the wafer is confined by some mechanism to anarrow angular distribution (e.g., the trajectories of the sputteredmaterial reaching the wafer are tightly distributed about a directionthat is normal to the surface of the wafer). A coherent depositioninsures that more sputtered material will reach, and deposit on, thebottom of the re-entry shaped contact holes. However, if the angle ofthe re-entrant shaped side walls is too great, even coherent sputteringoften cannot completely cover the walls of the holes or completely fillthe holes.

In other words, the commonly used sputter deposition and reflowprocesses are not well suited for reentry shaped contact holes.Specifically, the presence of re-entry shaped contact holes tends toresult in poor to nonexistent electrical contacts to the devices at thebottom of the contact hole. Consequently, a wafer conventionally must bescrapped if the process for etching contact holes inadvertantly producesoverly reentrant shaped holes. Thus, a need exists for a depositionprocess for coating the walls of, or filling, reentrant shaped holes, sothat wafers having such holes would not have to be scrapped.

SUMMARY OF THE INVENTION

The invention is a method and apparatus for depositing a layer ofmaterial on a semiconductor substrate whose surface includes holes. Theinvention facilitates conformally covering the walls of the holes, orelse completely filling the holes without voids, even if some of theholes are reentrant shaped, that is, have apertures smaller than theirbases.

A first aspect of the invention is a method for conformauly covering thewalls of the holes with a layer of material. The method includes thesteps of: (a) depositing the material onto the bottoms of the holes to afirst thickness less than that required to fill the holes; and (b)reverse sputtering the deposited material so as to completely coat theside walls of the holes with the deposited material.

Preferably, the reverse sputtering step (b) is performed concurrentlywith the deposition step (a) by applying a negative bias voltage to thesubstrate during the deposition.

The deposition step (a) preferably includes depositing the material"coherently", that is, directing the material toward the substrate intrajectories sufficiently perpendicular to the substrate that asubstantial portion of the material which enters the opening of eachhole deposits on the base of the hole. The coherent depositionpreferably is performed by sputtering the material (or a precursor ofthe material) from a sputtering target, while obtaining coherentsputtering trajectories either by positioning the substrate a sufficientdistance from the sputtering target, or by interposing a collimatorbetween the substrate and the target.

A second aspect of the invention is a method for completely filling theholes without voids. This method includes: (a) a first step ofconformally covering the walls of the holes in accordance with the firstaspect of the invention as defined above, and then (b) depositing ontothe substrate surface a second thickness of material sufficient to fillthe holes, while (c) simultaneously heating the substrate surfacesufficiently to enhance reflow of the material being deposited. Theinitial conformal coating functions as a "wetting" or "nucleation" layerduring the hole-filling deposition step. Consequently, materialdeposited on the upper portions of the walls of a hole during thehole-filling deposition step will flow over the wetting layer so as tocompletely fill the hole without voids. This contrasts with conventionaldeposition processes in which material deposited on the upper portionsof the walls of a hole can create an overhang that occludes the hole,thereby leaving an unfilled void within the hole.

The step of heating preferably includes both bombarding the surface ofthe substrate with electrons to enhance reflow of the deposited materialand heating the substrate from a heat source such as a resistivelyheated pedestal or radiant heat lamps. The bombarding electrons shouldhave energy levels large enough to heat the deposited material, but notso large as to damage electrical devices fabricated in the underlyingsubstrate.

Because the hole-filling process of the invention causes reflow of thematerial deposited on the outer surface of the substrate as well as thematerial deposited in the holes, the process advantageously planarizesthe material deposited on the surface of the substrate while the holesare being filled. In contrast, conventional processes for filling holesoften produce a layer on the outer surface of the substrate havingundulations conformally following the hole pattern, and theseundulations must be removed by an additional planarizing step. Anadvantage of the present invention is that it can eliminate the need forthis additional planarizing step:

The invention now makes it possible to fill high aspect ratio holes evenif the holes have a reentry shape, that is, an aperture smaller than thebase. Previously, when an etch process used to produce a holeinadvertandy produced reentry shaped holes, the entire wafer had to bescrapped because the existing deposition and planarization techniqueswere not able to reliably fill the contact holes without forming voidswithin the holes. Consequently, it was very important to tightly controlthe etch process so as to prevent the formation of re-entry shapedholes. The new metalization and planarization process in accordance withthe invention is much more tolerant of variations in the preceding etchprocess.

Other advantages and features will become apparent from the followingdescription of the preferred embodiments and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional reentry shaped contact hole.

FIG. 2 is a diagram of a sputtering deposition chamber in which theconformal deposition process can be carried out.

FIG. 3 is a diagram of a sputtering deposition chamber in which thehole-filling and planarizing process may be carried out.

FIG. 4 is a flow chart of a complete metalization process includinghole-filling and planarizing.

FIG. 5 is a diagram of a conventional cluster tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1.0 System Hardware

A hole-filling and film planarizing process in accordance with theinvention involves two deposition phases referred to as Phase I andPhase II. Phase I conformally coats or covers the side walls of theholes on the surface of a semiconductor wafer or substrate, and thenPhase II fills the holes. If it is desired to conformally cover thewalls of the holes without filling the holes, Phase I can be performedwithout Phase II.

During Phase I, a predetermined amount of material (e.g., Al) iscoherently sputter deposited onto an unheated semiconductor substrate orwafer. Either concurrently or subsequently, the surface of the substrateis bombarded with inert gas ions.

During Phase II, the substrate is heated and additional material isdeposited onto the substrate, preferably while the deposited material issimultaneously bombarded by electrons. In contrast with Phase I, thePhase II deposition process need not be coherent.

Phases I and II can be performed in a single chamber. However, in thepreferred embodiment described below, Phase I and Phase II are carriedout in separate chambers. The Phase I process preferably is carried outin a coherent deposition chamber which includes a mechanical collimator,as illustrated in FIG. 2. The Phase II deposition is carried out in aconventional deposition chamber without a collimator, as illustrated inFIG. 3. Before describing the details of each Phase, the two depositionsystems will be described with reference to features that are relevantto practicing the invention.

Referring to FIG. 2, a sputter deposition system in which the Phase I ofthe planarization process is carried out includes a deposition chamber10, a source assembly 12 on which a sputter target 14 is mounted, and amovable lower pedestal or platform 16 for holding a substrate 18 onwhich material sputtered from target 14 is deposited. In the describedembodiment, the source assembly is a magnetron, although anyconventional sputtering source can be used. The source assembly and thetarget mounted thereon are electrically isolated from the rest of thechamber by an insulator ring 20. The lower platform can be raised andlowered by a mechanical lift mechanism 22. The lift mechanism raises thesubstrate up until it contacts a clamping ring 24. The clamping ring,which has a central aperture 26 that is slightly smaller in diameterthan the substrate, holds the substrate onto the platform and shieldsthe platform from the plasma and deposition material during processing.

A collimation filter 40, preferably positioned midway between the targetand the substrate, separates chamber 10 into an upper cavity 42 and alower cavity 44. Sputtered material leaving the target typically has abroad range of trajectories that are distributed around a direction thatis normal to the target The collimation filter blocks (i.e., filtersout) all sputtered material having a trajectory that diverges from thenormal direction by more than a preselected angle. The collimationfilter is a metal plate, preferably made of a refractory material suchas titanium. It has an array of holes passing through it which have aspecified aspect ratio, which is defined as the ratio of their length totheir diameter. To maximize throughput, the holes are hexagonal in shapeand form honeycomb structure. The maximum angle from the normal throughwhich particles can travel through the collimator without being blockedapproximately equals the angle whose cotangent is the aspect ratio ofthe holes. For example, long, narrow holes filter out more of thesputtered material and produce a narrower angular distribution than doshort, wide holes. In the described embodiment, the plate has athickness of about 0.950 inch and the holes are about 0.625 inch indiameter.

The collimation filter preferably is connected to ground and thus formsa ground plane separating the two cavities. Within the upper cavity andaround its inside perimeter there is a cylindrical shield 46 thatprevents material from being deposited on the walls of the chamber.Likewise, the lower cavity also includes another cylindrical shield 48which serves a similar purpose. Both shields are connected to groundpotential.

A vacuum pump 28, connected to the chamber through a vacuum line 30, isused to evacuate the chamber to establish the appropriate vacuum at thebeginning of a process run. Gas control circuitry 32 controls the flowof inert sputtering gas (e.g. Ar) into the chamber during processing. ADC voltage supply 34 supplies power to the target to initiate andsustain a plasma deposition process. The negative terminal of the DCsupply is connected to the target and the positive terminal of the DCsupply is connected to the electrically grounded wall of the chamber. Asecond DC power supply 50 biases the platform, including the substrate,negatively relative to the collimation filter and the chamber wall.

Platform 16 includes a network of coolant passageways 52 through whichan external pump (not shown) circulates a coolant (e.g. water at 23° C.)to cool the platform during processing.

The deposition chamber just described is entirely conventional, exceptfor the inclusion of the second DC power supply 50 to negatively biasthe substrate. As will be described below, the negative substrate biascauses reverse sputtering of the material (e.g., TiN) being deposited onthe substrate.

A deposition chamber adapted for carrying out the Phase I deposition isshown in FIG. 3. This chamber has most of the same elements as thechamber shown in FIG. 2 (indicated by like numbered elements in thefigures). One principal difference is the absence of a collimationfilter. As will be explained below, the Phase II deposition is conductedunder conditions which promote reflow of the deposited material.Specifically, reflow occurs during Phase II because the holes alreadyhave been coated with the nucleation layer deposited during Phase I andthe substrate surface is heated sufficiently to induce reflow. Thereflow of material while it is deposited during Phase II prevents a holefrom being occluded by material deposited on the upper portions of thehole walls, even if the material is incoherent or uncollimated.

Since a collimator is not necessary during Phase II, it isadvantageously omitted, because a collimator would unnecessarily wasteexpensive target material by blocking some sputtered material fromreaching the substrate. Without a collimator, a given deposition ratecan be achieved at a reduced target sputtering rate compared with achamber having a collimator, thereby extending the life of the target.Of course, since the Phase II depostion chamber has no collimationfilter, it needs only one cylindrical shield 68 extending from near thetarget toward the platform.

Another principal difference between the deposition chambers used forPhases I and II is that the platform 16 in the Phase II chamber includesa resistive heater 70 which is operated by a heater control circuit 72to heat the platform to a user setable temperature (e.g. 520° C. for Aldeposition). A DC supply 73 is connected to platform 16 so as to biasthe substrate, as was possible in the chamber shown in FIG. 2. However,in this case the DC supply is connected so as to bias the substrate to apositive voltage relative to the chamber wall rather than to a negativevoltage as was the case for the coherent deposition chamber.

The DC supplies, the gas controls, the lift mechanism and the otherelectronic components in the system are controlled in conventionalfashion by a control module 71 shown in FIGS. 2 and 3. The controlmodule is programmed to automatically carry out the sequence ofoperations described herein thus minimizing the need for operatorinvolvement.

2.0 Planarization Process

The steps of the Phase I and Phase II depositions are shown in FIG. 4and described below.

2.1 Phase I

In the preferred embodiment, the Phase I conformal deposition ofaluminum is carried out in the chamber shown in FIG. 1 using an argonatmosphere. To perform the Phase I deposition and resputtering processessimultaneously, both the target 14 and the wafer support platform 16(and hence, the semiconductor wafer 18) are biased to negative voltagesso that both the target and the substrate are bombarded by ions from theplasma. The collimator 40 is electrically grounded. In addition,sufficient electrical power is coupled to the argon gas produce a plasmaThe source of power may be the bias voltage on the target, or it may bea separate, conventional plasma excitation source. Ar⁺ ions from theupper cavity plasma generates bombard the negatively biased Al targetwith sufficient energy to sputter Al atoms off of the target and towardsthe substrate (step 102).

As stated in the Summary of the Invention, the Phase I deposition andresputtering processes can be performed sequentially rather thansimultaneously, although this alternative presently is considered lesspreferable. In this alternative, the target would be negatively biasedonly during the deposition process, and the substrate would benegatively biased only during the subsquent resputtering process.

In the resputtering process, whether performed simultaneously or afterthe deposition, the bias applied to the wafer support pedestal orplatform 16 produces a second plasma in the lower cavity. This plasma,like the plasma produced in the upper cavity, generates Ar⁺ ions. Butbecause of the negative bias on the platform 16, the Ar⁺ ions generatedin the lower cavity accelerate towards and bombard the surface of thesubstrate while the Al sputtered off the target is also being depositedthere (step 104). The bombarding Ar⁺ ions serve to resputter thedeposited material at the bottom of the contact holes onto the sidewalls of the reentry shaped holes, thereby forming a nucleation layer orwetting layer on those side wall surfaces.

Increasing the negative bias voltage on the platform proportionatelyincreases the energy of the bombarding Ar⁺ ions, which increases therate at which the ions sputter material off of the bottom of the contactholes. The bias voltage should be sufficient to resputter some of the Almaterial being deposited on the bottom of the contact hole (i.e., aboveabout 20-30 eV), but not so high as to remove all of the material thatis being deposited on the bottom of the contact hole, and not so highthat the bombarding Ar⁺ ions damage any electrical devices in theunderlying substrate. We have successfully tested the invention using DCpower supply 73 to apply a bias of -450 volts to the wafer supportplatform 16. For depositing material other than aluminum, theappropriate bias voltage should be adjusted in proportion to thesputtering yield of the material. The sputtering yield of variousmaterials as a function of the energy of bombarding ions (and hence theplatform bias voltage) is extensively published, so the bias voltagereadily can be adjusted for depositing materials other than aluminum.

Preferably, the substrate is not heated during the resputtering process,so as to maintain the substrate at a relatively cool temperature.Specifically, the substrate temperature should be maintained low enoughto avoid two undesirable results. First, the temperature should be lowenough to prevent material resputtered from the base of the hole fromresputtering any substantial amount of material already deposited on theside walls rather than depositing there. Second, the temperature shouldbe low enough to prevent the material deposited on the side walls from"de-wetting", i.e., leaving some of the side wall surface uncovered bythe deposited material. Preferably, this is accomplished by maintainingthe substrate at a temperature no greater than about 150° C. In thepreferred embodiment, the platform 16 is water cooled during Phase I tomaintain the substrate temperature less than about 50° C. (step 100).

Instead of using a mechanical collimator during the Phase I depositionprocess, similar results can be achieved by increasing the distancebetween the target and the substrate (the "sputtering distance"),thereby increasing the coherency (i.e., reducing the maximum trajectoryangle) of the sputtered material. Whether the coherency of the sputteredmaterial is achieved by a collimator or by a long sputtering distance,the coherency must be sufficient to deposit a substantial amount ofmaterial on the base of each hole. The material deposited on the base ofeach hole is resputtered (by the resputtering process) onto the sidewalls of the hole to achieve complete, conformal coverage of the holewalls by the material. If the sputtered material is overly incoherent(i.e., an excessive portion of the material has oblique trajectoriesrelative to the substrate surface), the amount of material deposited onthe base of each hole will be insufficient to completely cover the sidewalls when it is resputtered. If a perforated plate collimator 40 isused to achieve coherency, as in the preferred embodiment shown in FIG.2, the coherency is increased by increasing the ratio of the length tothe width of the perforations. If coherency is achieved by a longsputtering distance, increasing the distance increases the coherency.

In the tested embodiment, the final desired metalization thickness wasabout 10,000 Å, and the Phase I deposition was continued until about4000 Å was deposited on the top surface of the substrate (step 106).This resulted in sufficient material deposited on the base of each holeto completely cover the sidewalls when resputtered. Of course, thethickness of material remaining on the base of each hole afterresputtering will be less than the thickness on the top surface of thesubstrate because some of the material originally deposited on the baseof a hole will have been resputtered onto the sidewalls of that hole. Inour tests using 0.35 micron wide holes with an aspect ratio of about3:1, the thickness of aluminum deposited on the bottom of the hole wasabout 40% of the amount that is deposited on the top surface of thesubstrate, namely, about 1600 Å.

2.2 Phase II

To perform the Phase II deposition, the substrate is transferred to thesecond sputter deposition chamber, shown in FIG. 2, of any conventionaldesign which preferably does not include a collimator. To protect thedeposited layer on the substrate from oxidation and contamination, thesubstrate preferably is maintained in a clean vacuum enclosure whilebeing transferred from the first chamber to the second chamber.

A second Al sputter deposition is performed in the second chamber (step108) to finish filling the holes. As in any conventional sputteringprocess, the target is biased to a negative voltage, so that argon ionsfrom the plasma bombard the target and sputter material from the targetonto the substrate.

During Phase II, the platform is heated to a temperature which is highenough to reflow the deposited metal (step 112). The Phase II depositionis continued until the deposited layer has the desired thickness (step114).

Preferably, during Phase II the reflow of the deposited metal isenhanced by applying a positive bias voltage to the platform 16. Thepositive bias on the substrate causes free electrons generated in thatplasma to accelerate towards and bombard the surface of the substratewhile Al is simultaneously being sputter deposited onto the substrate(step 110). The positive bias on the substrate is set sufficiently highto cause the bombarding electrons to heat the deposited Al material,thereby enhancing reflow of the material.

In the preferred embodiment, the deposited aluminum layer is heated toabout 600° C. to enhance reflow. Because electrons are bombarding thesubstrate and thereby heating the deposited Al material, the depositedAl layer will be at a higher temperature than the substrate.Consequently, the substrate temperature need not be as high as would berequired to achieve reflow in the absence of the electron bombardment Inthe preferred embodiment, the substrate is heated to only 450° C. toproduce an Al layer temperature of about 600° C. Thus, an advantage ofbombarding the substrate with electrons during deposition is that itreduces the substrate temperature required to reflow the depositedmaterial as compared to conventional reflow processes,. (Because ofimperfect heat conductivity between the substrate and the platform inthe tested embodiment, the platform is heated to 520° C. to raise thesubstrate temperature to this 450° C. level.) The energy of thebombarding electrons must be sufficient to heat the layer of depositedmaterial, but not so high as to penetrate through the deposited metaland into the underlying devices where they may cause damage. In general,it is desirable to maximize electron density by applying high power tothe plasma to maximize the plasma density, and it is desirable tominimize electron energy by applying a low bias voltage to the platform.

Because the sidewalls of the contact holes have been "wetted" (i.e.,coated by a "nucleation layer" of the material being deposited) by thereverse sputtering which took place during Phase I, during the Phase IIdeposition the reflowed metal will more readily flow into and fill thecontact holes. In contrast, in prior art process in which the sidewallsare not wetted, the deposited material can more readily form a bridgeover the opening of the contact hole and leave an unfilled hole beneaththe bridge.

In the described embodiment, during the Phase II deposition the targetvoltage is kept at the same -500 volt level used in Phase I, but thetarget power is reduced to about 2,000 watts, and the bias applied tothe platform is set to about +100 to +200 volts. The resultingdeposition rate is reduced as compared to the Phase I deposition. Thisreduced deposition rate provides sufficient time for the depositedmaterial to redistribute itself and fill the contact holes during thedeposition. If the target power is set too high, the deposition rate maybe so high that the deposited material may not be able to flow (i.e.,redistribute) quickly enough to prevent the small contact holes frombridging over.

As indicated above, it is preferable that the substrate be transferredfrom the first chamber to the second chamber under vacuum conditions.This is to avoid contamination of any kind from adhering to the surfaceof the recently deposited layer during the transition from Phase I toPhase II. If gas molecules, such as O₂ or N₂, are permitted to attach tothe new surface, they can negatively impact the adhesion and reflow ofdeposited material during the subsequent Phase II deposition.

An transfer under vacuum can easy be done in conventionally designed,commercially available deposition systems, such as the Centura 5200 PVDcluster tool that is sold by Applied Materials, Inc. of Santa Clara,Calif. FIG. 5 is a diagrammatic representation of a representativecluster tool. It typically includes a central transfer chamber 80 ontowhich can be bolted one or more process chambers, e.g. a depositionchamber 82 (i.e., the Phase I chamber) and a reflow camber 84 (i.e., thePhase II chamber). The process chambers and the transfer chamber areisolated from each other by slit valve openings, which can be opened toallow the transfer of substrates into and out of the chamber and closedto isolate the connected chambers from each other. Thus, each chambermay be maintained at its own vacuum pressure and atmosphere independentof the other chambers. In addition, the transfer chamber includes arobot mechanism 86 for transferring substrates into and out of thedifferent process chambers and from one chamber to the next chamber.Separate vacuum pumps and gas supply systems (not shown) are used toproduce the desired vacuums and process atmospheres in each chamber.

Both phases can, of course, be performed in the same chamber rather thantwo different chambers. In that case, the deposition chamber would havethe combined features of the chambers shown in FIGS. 2 and 3. Forexample, the platform would have cooling channels in it for circulatingcoolant to maintain the substrate at a cool temperature during the PhaseI deposition, and it would also have a resistive heater for heating thesubstrate during the Phase II deposition. Further, it would bepreferable that the power supply which is connected to the platform beswitchable so that the polarity of the bias to the platform may beeasily reversed when transitioning from a Phase I deposition to a PhaseII deposition.

In addition, the presence of a collimation filter during Phase II thatblocks the flow of electrons from the upper cavity plasma to theplatform, would require the formation of a second plasma in the lowercavity to provide a source of bombarding electrons. The second plasmacan be generated by applying a larger bias voltage to the platform or byproviding a separate source of electrical power to excite a plasma abovethe substrate. Alternatively, the collimator could be moved out of thepath between the target and the substrate during Phase II.

As stated in the description of Phase I, instead of using a mechanicalcollimator during Phase I, similar results can be achieved by increasingthe distance between the target and the substrate (the "sputteringdistance"), thereby increasing the coherency (i.e., reducing the maximumtrajectory angle) of the sputtered material. If a mechanical collimatoris not used, both Phase I and Phase II readily can be performed within asingle deposition chamber. In this case, the distance between the targetand the substrate preferably should be reduced during Phase II, becausegreater distances generally result in more sputtered material beingwasted by deposition on the chamber walls.

3.0 Experimental Results

The benefits of a Phase I deposition followed by a Phase II depositionare illustrated by the following experimental example of two aluminummetalizations which we performed. A two phase process was performed ontwo wafers, each of which had an oxide layer containing an array ofreentry shaped contact holes with an aspect ratio of about 3:1 and a topopening of about 0.3 microns. Both processes were identical except thatthe appropriate biases for conducting Phase I and Phase II typedepositions were applied to the platform for one wafer but not for theother.

Both wafers were prepared for the aluminum metalization by firstperforming a 200 Å preclean etch to produce a clean surface on the oxideand improve the contact resistance of the contacts that are to bedeposited. Then, using a coherent deposition process, a 200 Å layer ofTi was deposited, followed by a 700 Å layer of TiN. The Ti depositionestablishes a nucleation or wetting layer for the subsequent TiNdeposition. The TiN layer establishes a barrier for preventing Al fromdiffusing into the silicon when the substrate is later heated during thereflow'process. After the barrier layer was deposited, the wafer washeated in a separate metal anneal chamber to 650° C. in an atmosphere ofO₂ and N₂. Then a 500 Å layer of Ti was coherently deposited onto thewafer to provide a "glue" layer to which the aluminum can attach. Withthis last deposition, the wafers were ready for the metalization.

For both wafers, the aluminum was deposited onto the wafer in two steps.During the first step, a 4000 Å layer of Al/0.5% Cu was coherentlydeposited onto cold wafers (i.e., 50° C.). During the second step, usinganother deposition chamber from that used for the first step, a 6000 Ålayer of Al/0.5% Cu was deposited (non-coherently) onto heated wafers(i.e., 520° C.). The process conditions were identical during the firststep except that a 450 volt DC negative bias was applied to the platformholding the second wafer and was not applied to the platform holding thefirst wafer.

During the second step, again the process conditions were identicalexcept that a 230 volt positive bias (current limited to 13 amps) wasapplied to the platform holding the second wafer but not to the platformholding the first wafer.

The results were examined by cross-sectioning the processed wafers andthen viewing them under a scanning electron microscope at amagnification of about 40,000. In the case of the first wafer (i.e., thewafer processed using conventional techniques), the deposited metalbridged over the tops of the reentry shaped contact holes and no metalflowed into the holes. In contrast, in the case of the second waferwhich was processed in accordance with the invention, the depositedmetal had flowed into the reentry shaped contact holes, completelyfilling them up.

It should be noted that the above-described technique can also be usedto improve the sidewall coverage for any deposited material. Forexample, one can use the technique to obtain sidewall coverage of theabove-mentioned Ti glue layer that is deposited before the Almetalization.

4.0 Alternative Embodiment

As stated above, increasing the plasma density permits increasing thesputtering rate and/or decreasing the substrate bias voltage to reducethe risk of damage to the electrical devices on the substrate. Acapacitively coupled plasma source such as the magnetron source used inthe described preferred embodiment generally cannot achieve as high aplasma density as an inductively coupled plasma source. A suitabledesign using an antenna to couple migher amounts of RF power to theplasma so as to increase the plasma density is described in U.S. patentapplication, Ser. No. 08/145,744, entitled "Collimation Hardware with RFBias Rings to Enhance Sputter and/or Substrate Cavity Ion GenerationEfficiency", incorporated herein by reference. This design can increaseplasma density in the lower cavity, increase the sputtering efficiencyin the upper cavity and/or control the reverse sputtering rate in PhaseI

Referring to FIG. 2, the modifications involve using a ring antenna 51located inside the upper and/or a ring antenna 53 inside the lowercavity. The ring antennae are used to pump additional power into theplasma thereby increasing the efficiency of the process. For example,the upper antenna serves to increase sputtering efficiency. Whereas, thelower antenna serves to increase electron plasma density and/or to gaingreater control over the reverse sputtering process.

Both antennae 51 and 53 are coils with one or more turns. An RFgenerator 61 coupled to upper ring antenna 51 through an RF matchingnetwork 59 provides the RF power to the upper ring antenna. A second RFgenerator 57 coupled to lower ring antenna 53 through an RF matchingnetwork 55 provides the RF power to the lower ring antenna. In bothcases, an electrical connection is made to the antenna through afeed-through in the wall of the chamber and the other side of theantenna is electrically connected to ground through another feed-throughin the chamber wall.

Note that delivering RF power through the lower antenna does not affectthe sputter rate of the target, but it does increase the bombardmentenergy and ionization of the sputtered species onto the wafer. Thus, thelower antenna can also be used to optimize the bombardment energy tocontrol the characteristics of the deposited material and the barriercharacteristics.

Though the power generators are described as DC power sources, this ismerely for purposes of illustration. The invention is not limited toonly using those power sources but encompasses other power generators.For example, it may be desirable to use an RF power source for producingbias on the platform. One advantage of the RF power source is that it isless likely to produce the arcing that tends to be associated with DCsources. In addition, if a lower antenna is used to introduce power intothe lower chamber cavity, then it may not be necessary to separatelybias the platform. The plasma that is created by the lower antenna willproduce a positive bias on the platform.

Other embodiments are within the scope of the following claims.

What is claimed is:
 1. Apparatus for forming a planarized layer ofmaterial on a substrate which includes contact holes having sidewallsthat are formed within a surface of the substrate, comprising:adeposition chamber; a sputter target made of a constituent of saidmaterial, a platform for holding the substrate; a first power supplyconnected to the sputter target; a second Power supply connected to theplatform; a control module programmed to carry out the followingsteps:depositing a first thickness of the material onto the surface ofthe substrate to form deposited material; reverse sputtering thedeposited material so as to coat the sidewalls of the contact holes withthe deposited material; after the first thickness of said material isdeposited onto the surface of the substrate, depositing a secondthickness of said material onto the surface of the substrate; and whiledepositing the second thickness of said material onto the surface of thesubstrate, causing the second power supply to bias the substrate to apositive voltage relative to around so that the substrate is bombardedby electrons thereby heating the substrate to enhance reflow of thematerial being deposited.
 2. The apparatus of claim 1 wherein thecontrol module is programmed to carry out the steps of depositing thefirst material and reverse sputtering concurrently.
 3. The apparatus ofclaim 1 further comprising a collimation filter between said target andsaid platform.
 4. The apparatus of claim 1 further comprising a heaterthermally coupled to the platform so as to heat the substrate duringprocessing.
 5. The apparatus of claim 4 wherein the control module isprogrammed to cause the heater to heat the substrate while the substrateis being bombarded by electrons.